This invention relates generally to an improved unitized, in-line electron gun for color cathode ray tubes, and more specifically, to the secure mounting and alignment of the electrodes of such guns, and to the enhanced utility of the mounting structures.
The unitized, in-line electron gun generates three co-planar electron beams developed by the thermionic emission of heated cathodes arranged in line. The resulting beams are formed and spaced by a tandem succession of electrodes spaced along the central axis of the gun. The electrodes cause the beams to converge at multiple phosphor groups located on the faceplate of the color cathode ray tube. The prime objective in the design of such guns is to provide small spot size and enhanced resolution. To accomplish this objective, the electron gun electrodes and their field-forming surfaces must be as nearly as possible accurately positioned in exact parallelism, exact spacing, and in exact concentricity in relation to each other as a stable and coherent unit immune as far as is practicable from displacement due to shock, vibration, handling during production, thermal heating, or other causes.
The most common method of mounting electron gun electrodes is the process of embedding extensions of the metal parts of the gun electrodes, commonly called "claws," into one or more structural members extending in a direction parallel to the electron beam axis of the gun. These structural members, or "beads," (also called "pillars" or "multiform beads") are typically made of glass. The beads provide support; proper positioning, spacing and concentricity; and electrical isolation for the several gun electrodes. It is common practice to embed the electrode holding claws in the center line of the structural bead, and to use a narrow bead not much wider than the claws themselves. As a result of the embedding process, stress is often created in the material of the bead at the point of claw insertion. Also, as a result of mechanical, thermal, or other stress, hair-line cracks often occur in the bead, following a flaw line extending across the narrowest cross-section of the bead itself. This condition is apt to result in complete fracture of the bead and destruction of the utility of the entire gun assembly. Also, such stress-induced cracks, even if minor at first, may eventually spread and result in paths for destructive electrical arcing and/or degradation of gun performance resulting from shifting or other misalignment of gun parts after installation in the cathode ray tube.
Another common deficiency of the presently used structural bead is one that, while not so obvious as a catastrophic fracture of the bead material, may exert a deleterious but more subtle effect on gun performance. This statement applies especially to unitized, in-line type electron guns, and concerns the establishment and maintenance of critical dimensional spacing between gun electrodes, the concentricity of the apertures of beam forming electrodes, and the maintenance of parallelism between the facings of the electrodes. This problem is exacerbated by the close spacing required between the electrodes, and by the fact that in in-line, unitized gun structures, each common electrode is as wide as the aggregate width of three individual electrodes in an in-line, non-unitized gun. (As used herein, the "width" dimension of an electrode is its dimension in the plane passing through the central axis of the beams and orthogonal to the gun's central axis). The gaps between electrodes must be small enough to provide good stray field isolation and proper beam-forming fields. A typical example is disclosed in Hughes U.S. Pat. No. 3,873,879: the control and screen grid electrodes, whose claws are embedded in two narrow glass beads, are spaced 0.009 inch (0.225 mm) apart. Another example: In the unitized, in-line gun that is the subject of the present invention, the following exemplary specifications with regard to spacing, concentricity, and parallelism of the electrodes apply. The spacing between grids one and two is 0.008 inch .+-.0.0007 (0.20 mm .+-.0.017); spacing between other grids is 0.040 inch .+-.0.0015 (1.01 mm .+-.0.038). Concentricity tolerances between grids one, two, and three are 0.0005 inch (0.0127 mm); between grids four, five and six, .+-.0.001 inch (0.025 mm). Parallelism tolerance between grids one and two is .+-.0.0008 inch (0.020 mm); between other grids, .+-.0.0015 inch (0.038 mm).
It is obvious that any spatial displacement in any direction, or any tilting (or "wedging") of the electrodes one to the other due to unsound mounting and retention of the electrodes can result in deleterious changes in the critical spacings and precise co-axial alignments that are so necessary to ensure proper gun performance, both for the short term and for the entire operating lifetime of the gun.
The tendency of electrodes of in-line, unitized gun structures to move, tilt or wedge and so lose proper spacing, aperture concentricity, and parallelism can be attributed in general to the present mechanical design of such electrodes. In the present state of the art, it is customary to use single claws on opposite sides of each gun electrode, with the claws each embedded in the center of glass beads. The resulting abbreviated base line of support of the single claw leaves the electrodes susceptible to shifting or skewing under mechanical or other stress, with consequent degradation of gun performance.
A present common method of mounting the electrodes of an in-line gun is shown in the afore-mentioned Hughes U.S. Pat. No. 3,873,879, wherein two beads located on opposite sides of the gun, and narrow in relation to the gun electrodes, provide for the mechanical support, spacing, alignment and concentricity of the electrodes. The relative narrowness of the beads in relation to the dimensions of the electrodes is shown by reference number 23 of FIG. 8 of the subject patent, and the narrowness and close spacing of the embedded electrodes is shown also by other figures in the disclosure. In gun structures supported by two narrow beads the gun assembly is subject to torsional or twisting stress so that any quantum of parallel misalignment of the two supporting bead members is amplified by a "scissoring" effect. As a result, components located more distantly from the pivot point of the "scissors," are subject to an even greater displacement, especially with regard to aperture concentricity.
U.S. Pat. No. 3,239,078 issued to Johnson discloses an electrode mounting system especially for small, single-beam electron gun assemblies. Quoting Johnson (Column 2, lines 32-36):"the salient feature of this invention reside in a strap mounted on a beam-forming electrode such as a grid or focus anode and having two laterally positioned prongs for connection into a single rod located parallel to the beam." The Johnson electrode mounting means described would not appear to be suitable for application to unitized, in-line guns with their critical tolerance requirements. Neither would the Johnson electrode support system appear to be adaptable to unitization, a serious drawback in today's cost-conscious market.
U.S. Pat. No. 3,622,831 issued to Blumenberg describes a bead for a single gun used in a miniaturized indicia tube. This patent discloses the use of a single claw which spans the bead and engages it at the edge and central regions. The bead, although wide by single-gun standards, is relatively narrow and susceptible to fracture because of the establishment of a potential fracture line (stress concentration) across the entire width of the bead. The Blumenberg claw design in this case would not be desirable in the supporting of electrodes in a unitized, in-line electron gun.
Attempts have been made to resolve some of the problems inherent in the beading of in-line, non-unitized guns by provision of an integral beading structure on each side of the gun which is common to all three guns. U.S. Pat. No. 3,816,789 discloses in one embodiment a beading structure comprising two relatively narrow bead components, interconnected by a bridging member to support and space the electrodes. Claws on the discrete gun electrodes are embedded in the relatively narrow bead components and in the bridging member. A structure of this type suffers from the described tendencies to fracture, and in addition, due to the narrow region of embedment of the claw relative to the width of the electrodes, the electrodes are apt to move or skew relative to one another due to the flexing of the single metallic support pillars pressed into each bead. A second embodiment disclosed in this patent includes a narrow central bead with integral out-stretched arms which, together with the central bead, hold the discrete gun electrodes. This embodiment is plagued with the problems inherent in the described first embodiment.
It is one aspect of this present invention to ameliorate the problems described in the foregoing prior art examples by setting forth means to enhance fracture resistance of beads while at the same time increasing the strength of the beads in resisting the forces of torsion and deflection. It is another aspect to provide means to enhance resistance to forces that can skew or misalign the bead-supported electrodes in spatial relationship one to the other.